A three-dimensional profile measurement for a large-scale object is an advanced product-checking means as well as a foundation technology for modern reverse engineering, digitized designing and manufacturing. Therefore, the technology of three-dimensional profile measurement is extensively applied in the manufacturing field. Vision inspection technology, as one of nowadays advanced technologies, achieves great progress on the basis of maturity and perfection in technologies such as electronics, photoelectric detection, image processing and computer. With its increasing application in three-dimensional profile measurement, more and more scholars have been dedicating their research into the vision inspection technology field.
In practical engineering, in order to measure a whole three-dimensional profile of an object, the object is usually subdivided into many sub-areas. These sub-areas are respectively measured by a vision sensor from various viewpoints. The obtained three-dimensional data of the sub-areas are in local coordinate frames. So the three-dimensional data of the sub-areas must be unified to a global coordinate frame, and this process is called three-dimensional data registration.
At present, the three-dimensional data registration method, in common use, adopts a mark points method. First, a number of mark points are attached on the whole surface of a measured object, and one or more high precision baseline rulers are placed near the measured object. Second, a high resolution digital camera takes images of these mark points and baseline rulers from various viewpoints. Then, the extrinsic parameters of the digital camera from various viewpoints are calibrated by the mark points and baseline rulers, thus a global coordinate frame is established. At the same time, three-dimensional coordinates of all the mark points in the global coordinate frame are easily acquired. Additionally, the vision sensor measures three-dimensional coordinates of the mark points in the local coordinate frame when it measures the local sub-areas. Finally, all the local three-dimensional data are unified in the global coordinate frame through the three-dimensional coordinates of these mark points in the local and global coordinate frames, and then the three-dimensional data registration is completed. In foreign countries, the Tritop System, developed by the Gom company in Germany, is a typical product for three-dimensional vision measurement data registration using mark points. In China, Beijing Tianyuan 3D Science & Technology Co., Ltd. also develops similar products.
The disadvantages of the above-mentioned three-dimensional data registration method include the following:
Certain numbers of mark points should be attached on the surface of a measured object before measuring, which should be cleaned away during a time after measurement. The processes of attaching and cleaning mark points are fairly complicated and time-consuming.
For a measured object having a large degree of curvature, the attached mark points on the surface will be seriously deformed which will affect mark points extracting precision in images, or the mark points may not be extracted at all, or even the mark points can't be observed by the global measurement system.
High resolution digital cameras need to take images of all the mark points from various viewpoints, and the extrinsic parameters between adjacent viewpoints need to be calibrated step by step. Hence there exists accumulative errors. The disadvantages of this method lie in its complicated operation and its low efficiency.